Iowa Chapter Director
Join Date: Jul 2002
Location: Ames, Iowa
the stock 4.2 rods are the same length as the 96+ 3.8, they're longer than the previous years had so this did make them a little stronger, but I'd never put the kind of pwoer you're looking for into them, thats asking for trouble.
If you're dead set on a 4.2 stroke with a roots blower deffinetely look into some kind of custom billet crank, or some kind of strengthening procedures, I believe nitride is correct. The 4.2 stroke doesnt really add power as much as it does torque, the hp gain is about 5 and the tq gain is about 20. In my opinion, if you've already got an efficient blower the 4.2 stroke isnt worth the risk of breaking the crank, I'd personally go with a 3.8 supercoupe forged crank for durability and reassurance.
I've got a friend here in town with a 95 supercoupe, with his old ported S blower he pushed about 18psi on pump gas, his new Autorotor 1.7L pushed about 14psi (still on pump gas) but does it much more efficiently and cooler than the m90 ever did, and gained tremendous power from doing so. The low end improved decently, top end has improved dramatically, being as the autorotor is twin screw (claimed to be more efficient than even a turbo application) and can spin higher while compressing air at a cooler rate than the m90. If you're looking to spin the motor past 6 grand you'll never do it with an m90, its not going to happen, they are heat monsters for one, and secondly you'll max out the rev limits of the m90 and it will destroy itself.
And if you want to go to 6krpm with stock ford parts you're going to end up with a hole in the side of your block, I'd never do that without forged h-beams, forged pistons, and a very good rotating assembly balance.
Also, if you want to go past 5krpm you'll want a better torque converter that wont balloon and explode. Contact dirtyd0g on this board and he'll set you up better than any aftermarket converter at a cheaper price. Seeing as youre wanting torque at 3000rpm I'd assume you've already contemplated a high stall converter.
That local friend I mentioned earlier still needs a dynotune, but estimates around 300rwhp on stock size valves with his own port job. Big valve heads are great, and will add a lot of power, especially in a forced induction application, but if you're looking to save costs you could do without them. But you could do this and keep your torque, you would deffinetely want your reground cam spec'd for the larger valves, and the pricise valve sizes as well. Like I said its all in the cam. A larger intake valve with lots of intake lift from a cam would be awesome on a twin screwed sc application with a high stall converter.
1988 Notch Mustang: - 438W, direct port n2o, t56
2003 SVT Focus: - SCT X3 tuner